This invention relates generally to an ink jet printing head of the type used to project droplets of ink on a printing media for printing and more particularly to an ink jet printing head using a piezoelectric element to change the volume of a pressure chamber so as to eject an ink droplet from a nozzle. Ink jet heads of the ink-on-demand type have found wide use as they are simple in structure and can be equipped with multiple nozzles. Various processes for manufacturing such ink jet heads have been proposed and some of the processes have been practiced. The prior processes are however disadvantageous in that fabricated ink jet heads are costly and poor in durability. As an example, U.S. Pat. No. 4,189,734 describes manufacturing ink jet heads of photosensitive glass ceramic such as Photoceram produced by Corning Glass Corp. Ink jet heads of photosensitive glass are quite expensive to fabricate since adequate temperature control for the photosensitive glass cannot be achieved easily during the manufacturing steps. The photosensitive glass in the completed ink jet heads is opaque making it difficult to inspect the heads for the degree of bonding between a substrate and a vibration plate.
According to Swedish Patent No. 364,385 (which corresponds to U.S. Pat. No. 3,988,745), substrates of molded plastic are superimposed and fastened together by screws. Since the substrates themselves are inexpensive to fabricate and the cost of fastening them with the screws is small, the resultant ink jet heads are less costly to manufacture. For complete sealing of a pressure chamber and passageways leading to nozzles, the surfaces of each substrate should be finished to a high level of flatness, and substrates of molded plastic cannot attain the required planarity. With multi-nozzle heads having a multiplicity of nozzles, the nozzles are fixed by a small number of common screws, an arrangement which cannot completely seal all of the passageways. Those passageways which are remote from the fastening screws tend to communicate with each other as a piezoelectric element is mechanically distorted, since the substrates are not sufficiently pressed against each other at such passageways.
One expedient to eliminate the foregoing difficulty would be to bond the substrates defining passageways with an adhesive. The adhesive might however flow into and clog nozzles of a small cross-section. Also, the adhesive coated on the passageway walls would tend to come off with time under the influence of ink flowing through the passageways and block the nozzles.
Another process of manufacturing ink jet heads has been practiced which comprises the steps of narrowing the tip of a glass tube into a nozzle and covering the glass tube with a tubular piezoelectric element. With such a process, however, the tubular piezoelectric element is quite costly to fabricate. It is dimensionally difficult to put a multiplicity of nozzles in a highly compact arrangement. Furthermore, the greater is the number of nozzles to be incorporated into an ink jet head, the higher the cost of manufacture of the head becomes.
In summary, the ink jet heads heretofore proposed or put to use have been either expensive and reliable in operation or inexpensive but undependable in operation.
What is needed is an ink jet head which is inexpensive to produce as well as reliable in operation.